Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	$ 80,950	$ 202,054
Other investments, nonqualified retirement plan assets	4,970	5,063
Total assets measured at fair value	126,056	257,368
Sponsored Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	30,538	38,312
Equity Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	6,645	8,740
Sponsored Closed-End Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities available-for-sale	2,953	3,199
Nonqualified Retirement Plan Assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Other investments, nonqualified retirement plan assets	4,970	5,063
Level 1 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	80,950	202,054
Total assets measured at fair value	126,056	257,368
Level 1 [Member] | Sponsored Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	30,538	38,312
Level 1 [Member] | Equity Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	6,645	8,740
Level 1 [Member] | Sponsored Closed-End Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities available-for-sale	2,953	3,199
Level 1 [Member] | Nonqualified Retirement Plan Assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Other investments, nonqualified retirement plan assets	4,970	5,063
Level 2 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	0	0
Total assets measured at fair value	0	0
Level 2 [Member] | Sponsored Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	0	0
Level 2 [Member] | Equity Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	0	0
Level 2 [Member] | Sponsored Closed-End Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities available-for-sale	0	0
Level 2 [Member] | Nonqualified Retirement Plan Assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Other investments, nonqualified retirement plan assets	0	0
Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	0	0
Total assets measured at fair value	0	0
Level 3 [Member] | Sponsored Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	0	0
Level 3 [Member] | Equity Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	0	0
Level 3 [Member] | Sponsored Closed-End Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities available-for-sale	0	0
Level 3 [Member] | Nonqualified Retirement Plan Assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Other investments, nonqualified retirement plan assets	$ 0	$ 0